In conjunction with the 24 families of priests who were scheduled to serve in the temple for 2 week periods annually (see I Chronicles 24), there were also 24 groups of choir leaders who came for 2 weeks per year as well. These men evidently were responsible for working with the 4,000 musicians who were mentioned in I Chronicles 23:5. So, if those thousands were also divided into 24 groups then the temple orchestra would have had something over 150 active members in it at any given time. This means that all these men apparently had other duties throughout the remainder of each calendar year which would have kept them occupied between their allotted stints of ministry.
Each group had their particular specialty in music. Asaph prophesied musically... presumably with psalteries and cymbals... or as we might call them, guitars and drums. Jeduthun and his crew prophesied in thanks and praise to God too, with harps. Heman and his group of experts used horns in their musical preaching. Altogether, with Romamtiezer's dozen and all the rest of the musical leaders, there were 288 choir leaders... with a different 12 assigned to each 2 week period in every year.